Microfluidic devices are useful tools for the analysis of a variety of fluids, including chemical and biological fluids These devices are primarily composed of fluid transport channels—for example input and output channels, plus structured areas for sample diagnosis For effective processing of the fluid by the device, the fluid controllably passes through these channels
Various types of microfluidic devices are known The channel cross-section dimensions in a microfluidic device can vary widely, but may be anything from the millimetre scale to the nanometre scale Reference to microfluidics in this document is not restricted to micrometre scale devices, but includes both larger (millimetre) and smaller (nanometre) scale devices as is usual in the art
A basic form of a microfluidic device is based on continuous flow of the relevant fluids through the channels
A development of this basic form has the active fluid conveyed through the channels in droplets held in suspension by a functionally inert carrier liquid. Generally, the bulk of the devices described herein are digital, i.e droplet-based, microfluidic devices. In such devices, a droplet is formed of a first liquid, the droplet liquid, suspended immiscibly in a second liquid, the carrier liquid The droplet liquid and the carrier liquid should be selected to be immiscible over the relevant time scale needed for good functioning of the device as determined by factors such as transit time, storage time, and reaction time within the device Droplets are generally spherical, but in use the droplets may be distorted by forces or constrained by boundaries of the channel or other parts of the microstructured device, so other shapes may exist. A droplet in the context of a digital microfluidic device is therefore a contiguous volume of a fluid held in a carrier liquid, wherein the fluid and the carrier liquid are immiscible
Microfluidic devices may be made from a variety of substrate materials, including thermoplastic, glass and crystal In thermoplastic microfluidic devices, the channels can be formed by a variety of means, including injection moulding
Many of the functions in known microfluidic devices are controlled electronically via electrodes arranged in or adjacent to the flow channels These electrodes may be driven by an alternating current (AC) or a direct current (DC) as required.
The electrodes may be formed by known methods such as the above-mentioned positive pressure injection or by ink jet printing. The technique comprises the printing of the pattern using a conductive ink comprising conductive particles dispersed in a solvent, followed by a sintering or curing process to dry the ink by evaporating the solvent and fusing the conductive particles to form a non-disrupted metallic conductive pathway.
WO2007/081385A2 discloses various droplet-based microfluidic devices in which electrodes for performing electrical functions on fluids are located in channels adjacent to the flow channels. To form the electrodes, the channels are filled with a molten metal alloy This can be performed with positive pressure injection with a syringe to inject the molten metal alloy into the channels The metal alloy then cools and solidifies It is also disclosed that microscopic solder spheres or ultra-violet (UV) curable conductive ink can be used to form a barrier between the flow channel and the electrode channel in order to define the geometry of the metal alloy components. The prior art document also envisages forming the electrodes by lithographic patterning using indium tin oxide (ITO) or a metal such as platinum. The microfluidic devices can include a combination of both integrated metal alloy components and a patterned electrically conductive layer. For example, it is disclosed that an electrode pair may be made from a first electrode made from a patterned electrically conductive feature and the second electrode from an electrode channel filled with a metal alloy
Ink jet printing of conductive inks on thermoplastic materials is known, and is for example described in US 2009/0078915, WO 2004/068389, US 2006/0065897 and WO 2008/069565
Known ink compositions contain non-volatile solvents, particularly high boiling point polyols such as glycerol The boiling point of these polyols is typically 80 to 300° C., in some embodiments 100 to 200° C. These components act as humectants to prevent premature drying of the ink in the jetting nozzles to ensure reliability of the jetting process The sintering is normally a heating step which evaporates the solvent of the conductive ink and fuses the metal nanoparticles to form a conductive track The presence of the high boiling point liquids influences the temperature of the sintering however as any remaining organic component will impede a conductive pathway, thereby producing a product with lower and more variable conductivity. Higher sintering temperatures require a greater energy input and may damage thermoplastic substrates.
Accurate and precise placement of the actuating and sensing electrodes is critical to device performance. For example, in the case of electrodes of the type that terminate a controlled distance from the flow channel it is important to be able to define the electrode-to-flow-channel separation distance reproducibly both in terms of accuracy and precision. The separation distance may for example be in the range 10-100 micrometres It is desirable that this separation is extremely well controlled to conform to a specified distance. Moreover, it is desirable that the electrode-to-flow-channel separation can be made very small, so that the electrodes can be placed very close to the flow channel For active electrodes, i.e. those intended to be actuated with an applied voltage, closer proximity of the electrode terminating edge to the flow channel will generally allow smaller voltages to be used to achieve the same field gradient in the flow channel, i.e. the same functional effect for lower voltage. Moreover, any variance from specification in the placement position of the edges of the electrodes will lead to variance in the distance from the electrode edge to the flow channel, which in turn will lead to variance in the magnitude of the electric field that will be produced in the flow channel for a given applied voltage. Moreover, should such an electrode be mis-fabricated so that it spreads into the flow channel the device would fail, since there would be an electrical short circuit with the channel rather than the desired insulator separation provided by the thermoplastic, glass or other substrate material. Similar points will apply to the accuracy and sensitivity of electrodes arranged to form passive devices, such as inductive droplet sensors
The need to accurately and precisely define the electrode positioning relative to the flow channel is a reason why it is attractive to place electrodes in their own channels as proposed in WO2007/081385A2 Nevertheless, even with this approach there is scope for lack of containment of the electrode material and the use of subsequent processing steps that will not damage or discolour a thermoplastic substrate, especially in the example where UV curable conductive ink is used